1. Field of the Invention
The present invention relates generally to a giant magnetoresistance (GMR) multilayer device and methods for testing the properties of current perpendicular to the plane GMR multilayers.
2. Description of the Prior Art
Current perpendicular to plane (CPP)-GMR devices have potential use as random access memories for computers. This type of memory has the advantage of high reliability, nonvolatility in the event of power loss and an infinite useful lifetime. This system operates on the basis of the giant magnetoresistance phenomena, which results from the current passing through a magnetic multilayer material, whose magnetic layers are either aligned or anti-aligned, with a resulting change in resistance between the two states.
Much effort has been made to produce this type of GMR devices. For example, Prinz discloses in U.S. Pat. No. 5,477,482 a method of producing ultrahigh density nonvolatile ferromagnetic random access memories using GMR metal multilayers deposited upon semiconductor wafers. However, it has been difficult to control the quality of such wafers during their production for lack of a simple testing method. Typically, in order to test a CPP-GMR wafer, CPP resistance of the CPP-GMR wafer must be measured. GMR multilayer devices inherently have a very low electrical resistance. This is because the electrical current is propagated vertically through a metallic multilayer stack whose path length is short and cross-section is very large.
A GMR multilayer wafer typically consists of ten to twenty layers, each layer being a few nanometers thick, for a total path length of around 1000 xc3x85. Therefore, even at sub-micron area dimensions, the total resistance for this wafer may be only 1 ohm or less. If the quality of the deposited multilayer film is to be tested before actually patterning it into a finished device, attempting to measure the perpendicular resistance to determine whether the unpatterned wafer is within specifications would not be possible with any currently available techniques. That is because, for example, a six-inch diameter wafer would have a perpendicular resistance of 10xe2x88x929 ohm, which is too small a resistance to be measured using currently available measurement devices.
Unfortunately, carrying out a full processing or patterning sequence in order to determine if the unpatterned original multilayer wafer is acceptable, is not cost effective. Furthermore, if it were desirable to carry out a material optimization study on an unfinished wafer by varying the thickness or composition of the multilayer, a technique that can measure the performance of a GMR device performance without investing in the full processing of each layer would be preferred.
In addition, it should be noted that attempting to determine the performance of a GMR multilayer device from a simple current in plane (CIP) measurement is not practical for several reasons. First, it is not clear whether an optimum CPP material is also an optimum CIP material. Typically, a device using a CPP material may require a very smooth interface, whereas a device using CIP material may require a rough interface. Secondly, a device using a CPP material requires a very thick top and bottom layer of a good conductor, which will short out a CIP measurement of in the CPP material. Lastly, a meaningful GMR measurement in the multilayer of a GMR device requires that the device can be placed in a well-defined anti-aligned state for comparison with a totally aligned state. This is difficult to guarantee in a fully unpatterned GMR wafer without deliberately constructing the nonmagnetic space layer to have a thickness, which guarantees anti-ferromagnetic coupling. As a result, conventional CIP measurements are not useful for most CPP-GMR devices, and therefore, this is not a good solution to the problem.
Therefore, it is an objective of certain embodiments of the present invention to provide a method to test the quality of a CPP-GMR multilayer wafer without fabricating complicated devices on the wafer.
It is another objective of certain embodiments of the present invention to provide a method to test an unpatterned CPP-GMR multilayer wafer multilayer directly and quickly without employing a complicated patterning process on the unpatterned wafer.
In a first aspect, the present invention relates to a device prepared for testing the current perpendicular to the plane resistance of the device including: a substrate, a conductive base layer on the substrate, and a magnetic multilayer containing a magnetic material on the conductive base layer. The magnetic multilayer includes at least one ring-shaped first continuous portion of the magnetic multilayer surrounding a hole in the magnetic multilayer, and a second portion of the magnetic multilayer separated from the first continuous portion by a trench surrounding the first continuous portion of the magnetic multilayer.
In a second aspect, the present invention relates to a method for testing the properties of a device including a substrate, a conductive base layer on the substrate and a magnetic multilayer containing a magnetic material on the conductive base layer. The method includes the steps of: forming at least one hole in the magnetic multilayer and an isolation trench around the at least one hole to isolate at least one first continuous portion of the magnetic multilayer around the at least one hole from a second portion of the magnetic multilayer, and determining a property of the device by contacting a probe with the at least one first continuous portion of the magnetic multilayer, applying a predetermined current or voltage to the probe and measuring a current or voltage at the conductive base layer.
For a better understanding of the present invention, together with other and further objectives thereof, reference is made to the following description, taken in conjunction with the accompanying drawings.